Retropositional parasitism of SINEs on LINEs: Identification of SINEs and LINEs in elasmobranchs

Some previously unidentified short interspersed repetitive elements (SINEs) and long interspersed repetitive element (LINEs) were isolated from variou s higher elasmobranchs (sharks, skates, and rays) and characterized. These SINEs, members of the HE1 SINE family, were tRNA-derived and were widesprea d in higher elasmobranchs. The 3'-tail region of this SINE family was stron gly conserved among elasmobranchs. The LINEs, members of the HER1 LINE fami ly, encoded an amino acid sequence similar to that encoded by the chicken C R1 LINE family, and they contained a strongly conserved 3'-tail region in t he 3' untranslated region. This tail region of the HER1 LINE family was alm ost identical to that of the HE1 SINE family. Thus, the HEL SINE family and the HER1 LINE family provide a clear example of a pair of SINEs and LINEs that share the same tail region. Conservation of the secondary structures o f the tail regions, as well as of the nucleotide sequences, between the HE1 SINE family and HERL LINE family during evolution suggests that SINEs util ize the enzymatic machinery for retroposition of LINEs through the recognit ion of higher-order structures of the conserved 3'-tail region. A discussio n is presented of the parasitism of SINEs on LINEs during the evolution of these retroposons.